Stabilizing additives for thermochromic pigments

ABSTRACT

A microencapsulation process is improved by adding a stabilizing agent that contains one or more catalytic organo-metal oxide materials, such as metal soaps. This functions as a crosslinker by causing unsaturated bonds in the microcapsule walls to react, thereby stabilizing the microcapsules against the effects of additives to coatings that, otherwise, degrade the functionality of thermochromic or photochromic materials at the microcapsule core.

BACKGROUND

Thermochromic materials undergo color changes in response to thermalexposure. This is frequently discussed in context of a hysteresis plotwhere color is activated and deactivated by thermal cycling. Thishysteresis is shown, for example, in U.S. Pat. No. 4,442,429 issued toKotani et al. The materials may be designed for specific applications tohave little thermal separation between the different sides of the plot,or a large separation. Other aspects of performance design may includethe thermal activation temperature.

Thermochromic pigments are conventionally made by well-knownmanufacturing processes. Typically, a leuco dye, a developer, oil, and apolymer, such as melamine formaldehyde or urea formaldehyde, arecombined and agitated to create a very fine emulsion. The properties ofthe emulsion are such that the oil dye, and developer reside within apolymer formaldeyde capsule. Melamine formaldehyde is a thermo set resinsimilar to formica. The substance is very hard and will not break downat high temperature. This material is almost entirely insoluble in mostsolvents, but it is permeable. Thus, U.S. Pat. No. 5,591,255 issued toSmall et al. advances the art by advising to forego use of additivesthat may permeate the capsules, especially certain ketones, diols,aldehydes, amines and aromatic compounds.

Even so, emerging new applications for thermochromic pigments demand theuse of additives that interfere with functionality of the thermochromicpigments. By way of example, uses are emerging for thermochromicpigments that may be mixed with epoxy or polyester vehicles to formmetal deco coatings. If desired, as suggested by U.S. Pat. No. 4,425,161issued to Shibahashi et al., the thermochromic pigments may be used incombination with such polymers as, hydrocarbon resin, acryl resin, vinylacetate resin, halogen-containing resin, diene resin, polyester resin,polyamide resin, polyurethane resin, epoxy resin, melamine resin andpolyurea resin. This may necessitate the addition of auxiliary solventsincluding such diluents as high boiling point aromatic hydrocarbonsolvents, waxes, terpene oils and fluorocarbon oils, as well as solventsto improve the rheology of the coatings. Thermochromic pigments blendedwith these materials have typically a short shelf life. Moreover, thematerials in these vehicles may significantly alter the thermochromicbehavior of the pigments, such as by suppressing the color activationtemperature or varying the width of the hysteresis window.

Known processes for microencapsulation of thermochromic or photochromicmaterials include, for example, those disclosed in U.S. Pat. No.4,028,118 issued to Nakasuji, U.S. Pat. No. 4,425,161 issued to Kito etal., U.S. Pat. No. 4,425,161 issued to Shibahashi et al., U.S. Pat. No.4,902,604 issued to Yamaguchi et al., U.S. Pat. No. 5,350,633 issued toSumii et al., U.S. Pat. No. 5,503,781 issued to Sumii et al., U.S. Pat.No. 6,139,779 issued to Small et al., and U.S. Pat. No. 7,732,109 issuedto Senga et al., al. of which are incorporated by reference to the sameextent as though fully disclosed herein.

SUMMARY

The presently disclosed instrumentalities advance the art and overcomethe problems outlined above by providing improved thermochromic pigmentsthat are stabilized against the deleterious effects of chemicaladditives that heretofore have damaged the pigments.

In one aspect, a stabilizing agent is combined with a slurry thatcontains thermochromic after the microcapsules are formed and cured. Byway of example, this mixing may be done as the slurry is being mixedwith a resin that is to be applied as an external coating on metal, suchas an epoxy resin that may be applied to coiled aluminum stock formaking beverage cans. Other useful resins include polyester resins,styrene resins, acrylates, and any other synthetic resins.

The stabilizing agent may be a metallocene catalyst or transition metalbonded to organic moieties through oxygen linkages. Preferred forms ofthe stabilizing agent are transition metal soaps, or any othercarboxylic acid salt including a catalytic metal-oxygen moiety. Theorganic tail of these preferred materials improves solubility anddispersion. Zirconium 2-ethyl hexanoate is particularly preferred.

In one aspect, Formula (1) below shows the structure of a carboxylatesalt that may be used as described herein:

M^(n)(R)n;

where M is a metal as described above of oxidation state n; and R is acarboxylate having a carbon number ranging from five to fourteen. M ispreferably a transition metal. R is preferably a branched derivative ofhexanoic acid, such as 2-ethyl hexanoate.

In another aspect, a metallocene catalyst may be provided withtransitions metals bonded to oxygen, nitrogen, and/or halogen atoms.

It has been discovered that the materials described above have thesurprising effect of protecting the color activation temperature ofmicrocapsules in solvent based coatings. Without being bound by theory,it appears that these materials attack unsaturated multiple bonds,especially pi bonds, at the exterior surfaces of the microcapsules. Inthe absence of excess hydrogen, these bonds crosslink to make themicrocapsules less permeable to deleterious additives that are,otherwise, capable of degrading the functionality of the materialsforming the core of the microcapsules. In addition, the crosslinkingeffect protects the microcapsule without necessarily attackingunsaturated bonds at the surface of the microcapsules. Thus, thestabilizing agent may be combined with any form of commerciallyavailable microencapsulated thermochromic pigments to beneficiallyaffect the performance of the pigments in any formulation for anyintended environments of use. The microcapsules stabilized in thismanner may be used in all applications for thermochromic pigmentsincluding, for example, thermochromic inks, paints, and coatings, inaddition to cast or injected articles of manufacture. Moreover, thismanner of stabilizing the microcapsules has additional uses in othermicroencapsulated materials, such as scented microcapsules, andphotochromic microcapsules. The amount of stabilizing agent suitablyranges from 0.5% to 15% by weight of the slurry.

DETAILED DESCRIPTION

The following embodiments teach by way of example and not by limitation.

Example

Thermochromic microcapsules are incorporated into a solvent based highmolecular weight epoxy coating where the solvent package contains polarsolvents such as methyl isobutyl ketone, 2-butoxy ethanol, and n-butanolfor example. The coating is then cured at a temperature above 200 C fora duration of 10 seconds to 30 seconds. In the absence of thestabilizing agent, the temperature profile of the thermochromic systemis significantly repressed so that the full color development cannot beattained unless the coated sample is subjected to sub-zero temperaturesfor a period of time. By the addition of stabilizing additives such as amelamine resin (Cymel 303), metallic salts (zirconium ethyl hexanoate),and other agents which can accelerate crosslinking in the resin systemsurrounding the microcapsules, the microcapsule wall becomes more stablethus protecting the internal thermochromic properties. The effect isthat the cured coating does not show a temperature repression for thefull color development, even in the presence of the strongly polarsolvents which are known to be harmful to the thermochromic propertiesof microcapsules. The effect of the stabilizing additive can besignificant at concentrations of 0.5-15% by weight of the thermochromiccoating.

1. In a microencapsulating process that includes forming an emulsionthat has respective hydrophobic and hydrophillic phases, and thereaftercuring a polymer to microencapsulate the hydrophobic phase as a slurry,the improvement comprising: adding a stabilizing agent to the slurrywherein the stabilizing agent includes a metal-oxygen moiety that iscapable of stabilizing the cured microcapsule walls
 2. The process ofclaim 1, wherein the metal of the metal-oxygen moiety is a transitionmetal.
 3. The process of claim 1, wherein the metal of the metal-oxygenmoiety is zirconium.
 4. The process of claim 1, wherein the metal-oxygenmoiety is a soap.
 5. The process of claim 4, wherein the metal-oxygenmoiety is a transition metal soap.
 6. The process of claim 1, whereinthe metal-oxygen moiety is a zirconium-metal soap.
 7. The process ofclaim 1, wherein the metal-oxygen moiety is zirconium 2-ethylhexanoate.8. The process of claim 1, wherein the metal-oxygen moiety is added in arange from 0.5% to 15% by weight of the slurry.
 9. The process of claim1, wherein the polymer is selected to contain dominantly at least onemember from the group consisting of melamine formaldehyde polymers andurea formaldehyde polymers.
 10. The process of claim 1, wherein themicrocapsules are formulated as a thermochromic pigment.
 11. The processof claim 1, wherein the microcapsules are formulated as a photochromicpigment.
 12. The process of claim 1, wherein the microcapsules areformulated as scented microcapsules.
 13. A slurry including stabilizedmicrocapsules made by the process of claim
 1. 14. A slurry includingstabilized microcapsules made by the process of claim
 2. 15. A slurryincluding stabilized microcapsules made by the process of claim
 5. 16. Aslurry including stabilized microcapsules made by the process of claim7.
 17. A slurry including stabilized microcapsules made by the processof claim 8.